Ducted fan jet engines for aircraft applications have a core engine located within an engine casing or cowl. The engine fan blades and casing are further surrounded by a nacelle spaced from the casing to provide a bypass duct about the core engine through which compressed air is forced by the fan blades.
A cavity is defined between the engine casing and the nacelle. The difference in profile between the core engine and the nacelle results in a cavity of considerable volume, in which it is known to mount accessories such as sensors and electrical or electronic components so as to make efficient use of the available space.
The cavity in question may be referred to as the engine compartment or fire zone.
A gas turbine engine and, in particular, the combustors produce a significant amount of heat during use, which results in greatly elevated temperatures within the cavity. The accessories mounted within the cavity are heat sensitive to a greater or lesser extent and can be damaged or degraded at such prevailing operating temperatures. Whilst steps have been taken to produce accessories which can withstand such elevated temperatures without imminent failure, it is well-known that the increased temperatures can impact on the operational life of such components.
In view of the above problem, it is known to provide a ventilation system for the cavity by directing cooling air from the engine bypass onto the engine casing and/or components 28 mounted thereon. This is achieved by providing ventilation ducts which are radially oriented with respect to the engine axis so as to bleed bypass air into the cavity. Each duct typically enters into the cavity and may comprise a flow restriction at its exit so as to produce a fast flowing jet of air directed towards the relevant components on the engine casing with the aim of providing maximum cooling effect in the vicinity of the jet.
A number of jets are angularly spaced about the cavity such that a global direction of the ventilation flow within the cavity is axial towards the trailing or downstream end of the cavity where the ventilation flow exits the cavity into the engine exhaust.
The ventilation flow through the cavity represents a parasitic loss to the efficiency of the gas turbine engine, causing reduced engine thrust and increased fuel consumption.
It is an aim of the present invention to provide an improved cavity ventilation system, which can provide suitable cooling with improved efficiency.